Related to size of the file

Hi all, I need to get as "4" in "txtnumbytes" when i write data to the location "800" as well as when am reading it from the location "804", the size of the file should be converted to hex and put it back in "txtnumbytes". Need help, how do i do this...As of now i have just written the code just to take the characters from file and put it into the file size. 

 

if(openfiledialog1.showdialog() == DialogResult.OK)

{

textbox1.text = openfiledialog1.FileName;

long filesize = new FileInfo(textBox1.Text).Length;

textBox2.Text = filesize.ToString();

} 

 

 

Thank you in advance